CODEWITHSUNDEEP
c#regex
harshit
harshit

Reputation: 3846

What does this regex mean

What I really need to know is :

  1. What does (?( mean ?
  2. What does ?: mean ?

The regex I am trying to figure out is :

(notice the above mentioned symbols in the following regex)

(?(?=and )(and )|(blah))(?:[1][9]|[2][0])[0-9][0-9]

Upvotes: 7

Views: 2155

Answers (4)

Anirudha
Anirudha

Reputation: 32797

(?(?=and )(and )|(blah)) pattern is used like if-then-else like (?(expression)yes|no) i.e and would be matched if and is there else blah would be matched

(?:) is a non capturing group.So it would not be included in the group or be used as back-reference \1

So,

(?(?=and )(and )|(blah))(?:[1][9]|[2][0])[0-9][0-9]

would match

and 1900
blah2000
and 2012
blah2013

NOTE(it's all about the groups)

The samething can be achievend with this regex (and |blah)(?:[1][9]|[2][0])[0-9][0-9]. The only thing in which these regex differ is the number of groups formed.

So my regex would form 1 group which would contain either and or blah

Your regex would form no groups.It will form a group only if it matches blah..

Upvotes: 3

pogo
pogo

Reputation: 1550

?: is a non-capturing group. (?ifthen|else) is used to construct if, then expression.

You can read more about these here.

http://www.regular-expressions.info/conditional.html

Upvotes: 0

Tim Pietzcker
Tim Pietzcker

Reputation: 336128

(?:...)

is a non-capturing group. It works just like (...), but it doesn't create a backreference (\1 etc.) for later re-use.

(?(condition)true|else)

is a conditional which tries to match condition; if that succeeds, it will then try to match true, if not, it will try to match else.

This is a rarely seen regex construct because there are not too many use cases for it. In your case,

(?(?=and )(and )|(blah))

could have been rewritten as

(and |blah)

Upvotes: 2

Nickon
Nickon

Reputation: 10146

Here's a quick reference to some patterns:

.   Any character except newline.
\.  A period (and so on for \*, \(, \\, etc.)
^   The start of the string.
$   The end of the string.
\d,\w,\s    A digit, word character [A-Za-z0-9_], or whitespace.
\D,\W,\S    Anything except a digit, word character, or whitespace.
[abc]   Character a, b, or c.
[a-z]   a through z.
[^abc]  Any character except a, b, or c.
aa|bb   Either aa or bb.
?   Zero or one of the preceding element.
*   Zero or more of the preceding element.
+   One or more of the preceding element.
{n} Exactly n of the preceding element.
{n,}    n or more of the preceding element.
{m,n}   Between m and n of the preceding element.
??,*?,+?,
{n}?, etc.  Same as above, but as few as possible.
(expr)  Capture expr for use with \1, etc.
(?:expr)    Non-capturing group.
(?=expr)    Followed by expr.
(?!expr)    Not followed by expr.

The expression (?(?=and )(and )|(blah)) is an if-else expression:)

You can test reqular expressions here: Regexpal.com

Upvotes: 2

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